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John Mastoras

Senior Partner

John  Mastoras


T:+1 416.216.3905

John Mastoras is a senior partner in our Canadian employment and labour law group. He is known to clients as a passionate and dedicated advocate and business partner who provides innovative and practical advice.

John is frequently called upon to litigate highly contentious matters such as disruptive labour disputes and workplace mental health issues, and acts exclusively for employers in the broader area of labour and employment law. He regularly litigates and provides strategic advice in connection with labour arbitrations, labour relations board disputes, human rights proceedings, occupational health and safety charges, workers' compensation, labour injunctions and common law employment actions. His practice spans union and non-union workplaces for both federally and provincially regulated employers.

John is often in the forefront as chief spokesperson in collective bargaining and provides helpful counsel in this regard to a variety of employers. He routinely acts for a broad group of employer clients  and employer associations in various industries, including energy, pulp and paper, pharmaceutical, health care, insurance, longshoring, construction, retail, food processing, transportation, steel, automotive and manufacturing more generally.

For the past 20 years, John has taught at the University of Toronto's Centre for Industrial Relations and Human Resources (CIRHR), where he is an adjunct professor. He was conferred the Morley Gunderson Prize by the University of Toronto as a CIRHR graduate who combines outstanding professional achievement with significant service to the university.

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  • Education
    • LL.B., Osgoode Hall Law School, 1987
    • M.I.R., University of Toronto, 1984
    • B.A.(Hons.), University of Toronto, 1981
  • Admissions
    • Ontario 1989
  • Rankings and recognitions
    • Chambers Canada, Ontario: Employment and Labour, Chambers and Partners, 2018
    • Best Lawyers in Canada (2015-2017): Labour and Employment Law
    • Canadian Legal Lexpert Directory (2012-2017): recommended in Workers' Compensation - Employer
    • Canadian Legal Lexpert Directory (2016): recommended in Labour Relations - Management
    • Recipient of the 2009 Morley Gunderson Prize conferred by the University of Toronto’s Centre for Industrial Relations and Human Resources. The prize is awarded annually to a graduate of the Centre for Industrial Relations who combines outstanding professional achievement with significant service to the Centre.
    • PLC Which lawyer? (2011): Labour and employment
  • Speaking engagements
    • “The Road to Understanding Recent Labour and Employment Law Developments,” Ontario Trucking Association, February 15, 2007.
    • “The Disabled Employee: Accommodation and Frustration of Contract,” Ogilvy Renault, June 7, 2006.
    • “Managing Absenteeism and Disabilities in the Unionized Workplace,” ORC Worldwide, May 31, 2006.
    • “Labour/Employment Law Hot Button Issues - Overview of the Shifting Landscape,” Ontario Trucking Association, February 16, 2006.
    • “Employer Alert: Bill 144 Automatic Certification, Bill C-45 Criminalization of Health and Safety Laws, and More,” International Warehouse Logistics Association, May 25, 2005.
    • “Recent Developments in Labour Arbitration,” Innovative Responses to the Emerging Issues in Human Resources and Industrial Relations, Centre for Industrial Relations, October 4, 2002.
    • “Ten Paths to Enlightenment - Topical Legal Issues for Employer Executives,” Zen and the Art of Employee Maintenance: Young Presidents’ Organization, November 2001.
    • “Accommodating Religious Needs in the Workplace,” The Duty to Accommodate in the Unionized Workplace, Workchannel, Centre for Labour-Management Development, April 5, 2001.
    • “Monitoring and Surveillance,” Infonex on Human Rights and Privacy in the Workplace, January 1997.
    • “Recent Trends in Workers’ Compensation Adjudication: Stress and Environmental Illness,” Major Benefits Developments, Federated Press on Human Resource Executives Conference, September 1995.
  • Memberships and activities


    • Canadian Bar Association
    • Law Society of Upper Canada
    • The Advocates’ Society
    • HR Policy Association (US)  
    • American Bar Association